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| United States Patent |
5,753,011
|
|
Sircar
, et al.
|
May 19, 1998
|
Operation of staged adsorbent membranes
Abstract
A staged adsorbent membrane system is operated to separate a gas mixture
wherein more strongly adsorbed secondary components preferentially adsorb
and permeate through the adsorbent membrane in the first stage. Less
strongly adsorbed primary components are recovered therefrom in a
nonpermeate gas product stream. Preferably two stages are utilized wherein
the permeate gas from the first stage is introduced into the second stage
and the nonpermeate gas from the second stage is recycled to the first
stage as additional feed gas to increase the overall recovery and/or
purity of the nonpermeate gas product. The two-stage membrane system is
operated such that the ratio of the recovery of the primary component in
the first stage to the recovery of the primary component in the second
stage is less than about 1.0. The method is particularly useful for the
recovery of hydrogen from hydrogen-containing gas mixtures.
| Inventors:
|
Sircar; Shivaji (Wescosville, PA);
Parrillo; David Joseph (Fleetwood, PA)
|
| Assignee:
|
Air Products and Chemicals, Inc. (Allentown, PA)
|
| Appl. No.:
|
785497 |
| Filed:
|
January 17, 1997 |
| Current U.S. Class: |
95/45; 95/47; 95/49; 95/50; 95/51; 95/96; 95/115 |
| Intern'l Class: |
B01D 053/22; B01D 053/047 |
| Field of Search: |
95/45,47-55,96,98-106,114,115
|
References Cited
U.S. Patent Documents
| 3144313 | Aug., 1964 | Pfefferle | 55/16.
|
| 3307330 | Mar., 1967 | Niedzielski et al. | 95/47.
|
| 4130403 | Dec., 1978 | Cooley et al. | 95/49.
|
| 4264338 | Apr., 1981 | Null | 55/16.
|
| 4690695 | Sep., 1987 | Doshi | 95/55.
|
| 4701187 | Oct., 1987 | Choe et al. | 95/55.
|
| 4894068 | Jan., 1990 | Rice | 95/51.
|
| 5032148 | Jul., 1991 | Baker et al. | 95/50.
|
| 5064446 | Nov., 1991 | Kusuki et al. | 95/53.
|
| 5064447 | Nov., 1991 | Lee | 95/48.
|
| 5104425 | Apr., 1992 | Rao et al. | 55/16.
|
| 5185014 | Feb., 1993 | Prasad | 55/16.
|
| 5240472 | Aug., 1993 | Sircar | 95/52.
|
| 5259869 | Nov., 1993 | Auvil et al. | 95/52.
|
| 5314528 | May., 1994 | Monereau | 95/55.
|
| 5332424 | Jul., 1994 | Rao et al. | 95/47.
|
| 5354547 | Oct., 1994 | Rao et al. | 423/650.
|
| 5378263 | Jan., 1995 | Prasad | 95/45.
|
| 5401300 | Mar., 1995 | Lokhandwala et al. | 95/49.
|
| 5407466 | Apr., 1995 | Lokhandwala et al. | 95/49.
|
| 5407467 | Apr., 1995 | Lokhandwala et al. | 95/49.
|
| 5415681 | May., 1995 | Baker | 95/45.
|
| 5431864 | Jul., 1995 | Rao et al. | 264/29.
|
| 5435836 | Jul., 1995 | Anand et al. | 95/45.
|
| 5447555 | Sep., 1995 | Yee et al. | 95/54.
|
| 5447559 | Sep., 1995 | Rao et al. | 96/4.
|
| 5507856 | Apr., 1996 | Rao et al. | 95/50.
|
| 5507860 | Apr., 1996 | Rao et al. | 96/12.
|
| 5556449 | Sep., 1996 | Baker et al. | 95/49.
|
Other References
T. Pettersen et al., "Design Studies of Membrane Permeator Process for Gas
Separation" in Gas Sep. Purif., vol. 9, No. 3, pp. 151-169 (1995).
N. I. Laguntsov et al., "The Use of Recycle Permeator Systems for Gas
Mixture Separation" in The Journal of Membrane Science, 67(1992), pp.
15-28.
J. Xu et al, "Gas Separation Membrane Cascades I. One-Compressor Cascades
with Minimal Exergy Loss Due to Mixing" in the Journal of Membrane
Science, 112(1996), pp. 115-128.
J. Xu et al, "Gas Separation Membrane Cascades II. Two-Compressor Cascades"
in the Journal of Membrane Science, 112 (1996), pp. 129-146.
R. W. Baker et al, "Recovery of Hydrocarbons from Polyolefin Vent Streams
Using Membrane Technology", AIChE Spring National Meeting, Feb. 25-29,
1996, New Orleans.
R. W. Spillman et al, "Economics of Gas Separation Membranes" in Chemical
Engineering Progress, Jan. 1989, pp. 41-62.
B. H. Bhide et al, "Membrane Processes for the Removal of Acid Gases from
Natural Gas. I. Process Configurations and Optimization of Operating
Conditions" in Journal of Membrane Science 81 (1993), pp. 209-237.
B. H. Bhide et al, "A New Evaluation of Membrane Processes for the
Oxygen-Enrichment of Air.I. Identification of Optimum Operating Conditions
and Process Configuration" in Journal of Membrane Science, 62 (1991), pp.
13-35.
S. A. Stern et al, "Recycle and Multimembrane Permeators for Gas
Separations", in Journal of Membrane Science, 20 (1984) pp. 25-43.
B. H. Bhide et al, "Membrane Processes for the Removal of Acid Gases from
Natural Gas. II. Effects of Operating Conditions, Economic Parameters, and
Membrane Properties" in Journal of Membrane Science 81 (1993), pp.
239-252.
B. H. Bhide et al, "A New Evaluation of Membrane Processes for the
Oxygen-Enrichment of Air. II. Effects of Economic Parameters and Membrane
Properties" in Journal of Membrane Science, 62 (1991), pp. 37-58.
|
Primary Examiner: Spitzer; Robert
Attorney, Agent or Firm: Fernbacher; John M.
Claims
We claim:
1. A method for separating a multicomponent gas mixture comprising at least
one primary component and at least one secondary component into a product
stream enriched in the primary component and one or more additional
product streams enriched in the secondary component, wherein the method
comprises:
(a) introducing a feed gas mixture comprising the primary and secondary
components into a first membrane separation zone having a permeable
adsorbent membrane comprising adsorbent material which divides the zone
into a feed side and a permeate side, wherein the gas passes through the
feed side of the membrane separation zone and a portion of the secondary
component is selectively adsorbed by the adsorbent material and permeates
through the membrane to the permeate side;
(b) withdrawing from the feed side of the first membrane separation zone a
first nonpermeate stream as the product stream enriched in the primary
component;
(c) withdrawing from the permeate side of the first membrane separation
zone a permeate stream which is enriched in the secondary component;
(d) introducing at least a portion of the permeate stream as a feed gas
into a second membrane separation zone having a permeable adsorbent
membrane comprising adsorbent material which divides the zone into a feed
side and a permeate side, wherein the feed gas passes through the feed
side of the membrane separation zone and a portion of the secondary
component is selectively adsorbed by the adsorbent material and permeates
through the membrane to the permeate side, and withdrawing therefrom an
additional product stream enriched in the secondary component;
(e) withdrawing from the feed side of the second membrane separation zone a
second nonpermeate stream which is further enriched in the primary
component, and combining at least a portion of the second nonpermeate
stream with the multicomponent gas mixture to provide the feed gas to the
first membrane separation zone; and
(f) operating the first and second membrane separation zones such that the
ratio of the recovery of the primary component in the first membrane
separation zone to the recovery of the primary component in the second
membrane separation zone is less than about 1.0, wherein the recovery of
the primary component in any membrane separation zone is defined as the
molar flow rate of the primary component in the nonpermeate stream
withdrawn from the zone divided by the molar flow rate of the primary
component in the feed gas introduced into the zone.
2. The method of claim 1 wherein the ratio of the recovery of the primary
component in the first membrane separation zone to the recovery of the
primary component in the second membrane separation zone is less than
about 0.8.
3. The method of claim 1 wherein the recovery of the primary component in
the first membrane separation zone is less than about 75%.
4. The method of claim 1 wherein the pressure of the feed gas mixture to
the first membrane separation zone is equal to or greater than the
pressure of the feed gas to the second membrane separation zone.
5. The method of claim 4 wherein the pressure of the feed gas mixture to
the first membrane separation zone is at least 30 psia.
6. The method of claim 1 which further comprises compressing the feed gas
prior to the second membrane separation zone.
7. The method of claim 1 which further comprises compressing at least a
portion of the second nonpermeate stream prior to combining with the
multicomponent gas mixture to provide the feed gas to the first membrane
separation zone.
8. The method of claim 1 wherein hydrogen is the primary component and a
hydrocarbon having from one to five carbon atoms is the secondary
component.
9. The method of claim 1 wherein hydrogen is the primary component and a
component selected from the group consisting of carbon oxides and methane
is the secondary component.
10. The method of claim 1 wherein hydrogen is the primary component and a
component selected from the group consisting of ammonia, nitrogen, and
hydrogen sulfide is the secondary component.
11. The method of claim 1 wherein the adsorbent material used in the first
membrane separation zone is selected from the group consisting of
activated carbon, zeolite, activated alumina, silica, and combinations
thereof.
12. The method of claim 11 wherein the adsorbent material used in the
second membrane separation zone is selected from the group consisting of
activated carbon, zeolite, activated alumina, silica, and combinations
thereof.
13. The method of claim 1 which further comprises introducing the product
stream enriched in the primary component into a pressure swing adsorption
system and withdrawing therefrom a final product stream which is further
enriched in the primary component.
14. The method of claim 1 which further comprises introducing the product
stream enriched in the primary component into a thermal swing adsorption
system and withdrawing therefrom a stream which is further enriched in the
primary component.
15. The method of claim 14 which further comprises introducing the stream
which is further enriched in the primary component into a pressure swing
adsorption system and withdrawing therefrom a final product stream which
is still further enriched in the primary component.
16. In a method for separating a multicomponent gas mixture comprising at
least one primary component and at least one secondary component into a
product stream enriched in the primary component and one or more
additional product streams enriched in the secondary component, wherein
the method includes:
(a) introducing a feed gas mixture comprising the primary and secondary
components into a first membrane separation zone having a gas permeable
membrane which divides the zone into a feed side and a permeate side,
wherein the gas passes through the feed side of the membrane separation
zone and a portion of the secondary component selectively permeates
through the membrane to the permeate side;
(b) withdrawing from the feed side of the first membrane separation zone a
first nonpermeate stream as the product stream enriched in the primary
component;
(c) withdrawing from the permeate side of the first membrane separation
zone a permeate stream which is enriched in the secondary component;
(d) introducing at least a portion of the permeate stream as a feed gas
into a second membrane separation zone having a gas permeable membrane
which divides the zone into a feed side and a permeate side, wherein the
feed gas passes through the feed side of the membrane separation zone and
a portion of the secondary component selectively permeates through the
membrane to the permeate side, and withdrawing therefrom an additional
product stream enriched in the secondary component; and
(e) withdrawing from the feed side of the second membrane separation zone a
second nonpermeate stream which is further enriched in the primary
component, and combining at least a portion of the second nonpermeate
stream with the multicomponent gas mixture to provide the feed gas to the
first membrane separation zone;
the improvement which comprises utilizing an adsorbent membrane comprising
adsorbent material as the gas permeable membrane in each of the first and
second membrane separation zones in which a portion of the secondary
component is selectively adsorbed from the feed gas to each zone by the
adsorbent material and permeates through the adsorbent membrane to the
permeate side of each zone, and operating the first and second membrane
separation zones such that the ratio of the recovery of the primary
component in the first membrane separation zone to the recovery of the
primary component in the second membrane separation zone is less than
about 1.0, wherein the recovery of the primary component in any membrane
separation zone is defined as the molar flow rate of the primary component
in the nonpermeate stream withdrawn from the zone divided by the molar
flow rate of the primary component in the feed gas introduced into the
zone.
17. The method of claim 16 wherein the ratio of the recovery of the primary
component in the first membrane separation zone to the recovery of the
primary component in the second membrane separation zone is less than
about 0.8.
18. The method of claim 16 wherein the recovery of the primary component in
the first membrane separation zone is less than about 75%.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
The invention disclosed herein was developed under DOE Cooperative
Agreement DE-FC04-93AL94461.
BACKGROUND OF THE INVENTION
Gas mixtures can be separated efficiently by the use of permeable membranes
in which the more permeable components are selectively recovered in an
enriched permeate gas and the less permeable components are recovered in
an enriched nonpermeate gas. The extent of enrichment depends on the
relative permeabilities of the components in the gas mixture as well as
the membrane module design and operating conditions. Different types of
gas-permeable membranes can be utilized for such separations including
polymeric membranes, porous inorganic membranes having controlled Knudsen
diffusion or molecular sieving properties, porous adsorbent membranes,
chemically active facilitated transport membranes, and high temperature
ion-conducting membranes.
The concept of staging to improve the performance of certain types of
permeable membrane systems is well known in the art. Many staging
configurations have been proposed and some have been used for the
separation of commercially-important gas mixtures. An important and
well-known example of effective membrane staging is the separation of
uranium isotopes from mixtures of gaseous uranium compounds in
multiple-stage gaseous diffusion columns using porous membranes which
effect gas separation based on Knudsen diffusion.
A general review and analysis of single- and multiple-stage membrane
permeator processes are reported by T. Pettersen and K. M. Lien in an
article entitled "Design Studies of Membrane Permeator Processes for Gas
Separation" in Gas Sep. Purif., Vol. 9, No. 3, pp. 151-169 (1995). Various
design methods and performance characteristics are presented for
single-stage and multiple-stage membrane permeation systems which can be
used for a variety of gas separations. Two generic types of multiple-stage
membrane configurations are described in which either the permeate or
nonpermeate stream from a membrane stage is further concentrated in a
second membrane stage and the further concentrated stream is recycled to
the feed of the first membrane stage. When the membrane permeate contains
the desired product component, product purity and/or recovery can be
increased by passing the nonpermeate stream to a second membrane stage and
recycling the permeate stream from this second membrane stage to the feed
of the first membrane stage. When the membrane nonpermeate stream contains
the desired product component, product purity and/or recovery can be
increased by passing the permeate stream to a second membrane stage and
recycling the nonpermeate stream from this second membrane stage to the
feed of the first membrane stage.
Methods for analyzing the performance of recycle permeator systems are set
forth in a paper entitled "The Use of Recycle Permeator Systems for Gas
Mixture Separation" by N. I. Laguntsov et al in the Journal of Membrane
Science, 67(1992), pp. 15-28. A two-stage permeator system is presented
and analyzed in which the permeate stream from the first stage is
introduced into a second stage and the nonpermeate from the second stage
is recycled to the first stage feed.
Additional analyses and design methods for multiple-stage membrane
separation systems are reviewed in an article entitled "Gas Separation
Membrane Cascades I. One-Compressor Cascades with Minimal Exergy Loss Due
to Mixing" by J. Xu and R. Agrawal in the Journal of Membrane Science,
112(1996), pp. 115-128 and a related article entitled "Gas Separation
Membrane Cascades II. Two-Compressor Cascades" by R. Agrawal and J. Xu in
the Journal of Membrane Science, 112(1996), pp. 129-146. These two review
articles disclose multiple-stage membrane arrangements, one of which is a
two-stage system wherein the permeate stream from the first stage is
introduced into a second stage and the nonpermeate from the second stage
is recycled to the first stage feed.
A paper by R. W. Baker et al entitled "Recovery of Hydrocarbons from
Polyolefin Vent Streams Using Membrane Technology" presented at the AlChE
Spring National Meeting, Feb. 25-29, 1996, New Orleans, discloses methods
of recovering hydrocarbons from nitrogen purge streams including the use
of a two-stage polymeric membrane system in which the permeate stream from
the first stage is introduced into a second stage and the nonpermeate from
the second stage is recycled to the first stage feed.
Much of the recent art in the staging of membrane separation systems
addresses the use of polymeric membranes. A review article by R. W.
Spillman entitled "Economics of Gas Separation Membranes" in Chemical
Engineering Progress, January 1989, pp. 41-62 describes the use of
polymeric membrane systems for recovering several commercially-important
gas products and the application of staging to these membrane systems. The
article also describes the integration of membrane systems with
absorption, distillation, and adsorption systems for the most efficient
recovery of specific gas products.
The recovery of methane from mixtures of methane, carbon dioxide, and
hydrogen sulfide by polymeric membranes is described in related U.S. Pat.
Nos. 5,556,449, 5,401,300, 5,407,466, and 5,407,467. The latter two of
these patents disclose a two-stage membrane system in which the carbon
dioxide- and hydrogen sulfide-enriched permeate stream from the first
stage is introduced into a second stage and the methane-enriched
nonpermeate from the second stage is recycled to the first stage feed.
Methane recoveries across individual stages are greater than 75%, with
first stage recoveries being lower than second stage recoveries and ratios
of first stage recovery to second stage recovery being 0.80 and above.
The recovery of methane from mixtures of methane and carbon dioxide by
staged polymeric membrane systems is described in a paper by B. D. Bhide
and S. A. Stern entitled "Membrane Processes for the Removal of Acid Gases
from Natural Gas. I. Process Configurations and Optimization of Operating
Conditions" in Journal of Membrane Science 81 (1993), pp. 209-237. Process
calculations and optimization studies are presented for a two-stage
membrane system in which the carbon dioxide-enriched permeate stream from
the first stage is introduced into a second stage and some or all of the
methane-enriched nonpermeate from the second stage is recycled to the
first stage feed. The use of a similar staged membrane system for the
oxygen-enrichment of air is described in a paper by B. D. Bhide and S. A.
Stern entitled "A new Evaluation of Membrane Processes for the
Oxygen-enrichment of Air. I. Identification of Optimum Operating
Conditions and Process Configuration" in Journal of Membrane Science 62
(1991), pp. 13-35.
Adsorbent membranes are used to separate gas mixtures by the combined
mechanisms of adsorption and surface diffusion using a thin layer of
nanoporous adsorptive material on a meso-macroporous support, wherein the
adsorptive material selectively adsorbs and permeates the more strongly
adsorbed components. The properties and methods of making these membranes
are disclosed in U.S. Pat. Nos. 5,104,425, 5,431,864, and 5,507,860.
Adsorbent membranes can be integrated with other separation processes such
as pressure swing adsorption and vapor-liquid fractionation as described
in U.S. Pat. Nos. 5,507,856, 5,332,424, 5,354,547, 5,435,836, and
5,447,559.
The preferred membrane staging configuration for a given gas separation
depends upon numerous factors such as (1) whether the desired product
component in the feed gas is recovered in the permeate or nonpermeate
stream; (2) the required product purity; (3) the relative value of the
product, which determines the acceptable recovery; (4) the tradeoff
between membrane capital cost and the cost of gas compression; and (5) how
the membrane separation step is integrated with other separation steps
such as distillation, adsorption, and absorption. In addition, the
separation properties of the membrane material can impact the staging
configuration and process design.
The gas separation characteristics of adsorbent membranes are markedly
different than those of polymeric membranes because the gas-solid
interactions and diffusion mechanisms which occur in the two membranes are
fundamentally different. For a given gas mixture, the components will
selectively permeate an adsorptive membrane in a different manner than a
polymeric membrane. For example, in the recovery of hydrogen from a
hydrogen-hydrocarbon gas mixture, the hydrogen selectively permeates a
polymeric membrane and is recovered as a hydrogen-enriched low-pressure
permeate stream. In contrast, with an adsorbent membrane the hydrocarbons
preferentially permeate and the hydrogen-enriched product stream is
recovered as the high-pressure nonpermeate stream. Because of these
fundamental differences between adsorbent and polymeric membranes, methods
for designing and optimizing multistage polymeric membrane systems do not
necessarily apply to multistage adsorbent membrane systems.
In the separation of commercially important gas mixtures by membrane
systems, a balance between product recovery and product purity must be
achieved at acceptable capital and operating costs. In staged membrane
systems, the stage configuration and operating conditions of individual
stages are important factors in meeting these purity, recovery, and cost
requirements. The invention described in the present specification and
defined by the claims which follow is an efficient method of operating
staged adsorbent membrane systems to achieve these objectives.
BRIEF SUMMARY OF THE INVENTION
The present invention is a method for separating a multicomponent gas
mixture comprising at least one primary component and at least one
secondary component into a product stream enriched in the primary
component and one or more additional product streams enriched in the
secondary component. The method comprises introducing a feed gas mixture
comprising the primary and secondary components into a first membrane
separation zone having a permeable adsorbent membrane which divides the
zone into a feed side and a permeate side, wherein the gas passes through
the feed side of the membrane separation zone and a portion of the
secondary component selectively adsorbs and permeates through the membrane
to the permeate side. The product stream enriched in the primary component
is withdrawn from the feed side of the first membrane separation zone as a
first nonpermeate stream, and a permeate stream which is enriched in the
secondary component is withdrawn from the permeate side of the first
membrane separation zone.
At least a portion of the permeate stream is introduced as a feed gas into
a second membrane separation zone having a permeable adsorbent membrane
which divides the zone into a feed side and a permeate side, wherein the
feed gas passes through the feed side of the membrane separation zone and
a portion of the secondary component selectively adsorbs and permeates
through the membrane to the permeate side. An additional product stream
enriched in the secondary component is withdrawn therefrom, and a second
nonpermeate stream which is further enriched in the primary component is
withdrawn from the feed side of the second membrane separation zone. At
least a portion of the second nonpermeate stream is combined with the
multicomponent gas mixture to provide the feed gas to the first membrane
separation zone.
The first and second membrane separation zones are operated such that the
ratio of the recovery of the primary component in the first membrane
separation zone to the recovery of the primary component in the second
membrane separation zone is less than about 1.0 and preferably less than
about 0.8, wherein the recovery of the primary component in any membrane
separation zone is defined as the molar flow rate of the primary component
in the nonpermeate stream withdrawn from the zone divided by the molar
flow rate of the primary component in the feed gas introduced into the
zone.
The recovery of the primary component in the first membrane separation zone
is preferably less than about 75%. The pressure of the feed gas mixture to
the first membrane separation zone preferably is greater than or equal to
the pressure of the feed gas to the second membrane separation zone, and
the pressure of the feed gas mixture to the first membrane separation zone
is typically at least 30 psia. The feed gas to the second membrane
separation zone may be compressed as necessary. At least a portion of the
second nonpermeate stream may be compressed prior to combining with the
multicomponent gas mixture to provide the feed gas to the first membrane
separation zone.
The multicomponent gas mixture may comprise hydrogen as a primary component
and one or more secondary components selected from the group consisting of
hydrocarbons having from one to five carbon atoms. Alternatively, when
hydrogen is a primary component, the one or more secondary components can
be selected from the group consisting of carbon oxides, methane, ammonia,
nitrogen, and hydrogen sulfide.
The stream from the membrane separation system which is enriched in the
primary component optionally is passed into a pressure swing adsorption
system and a final product stream which is further enriched in the primary
component is withdrawn therefrom. Alternatively, the stream from the
membrane system which is enriched in the primary component can be
introduced into a thermal swing adsorption system and a stream which is
further enriched in the primary component withdrawn therefrom. In another
optional operating mode, the stream from the membrane system which is
further enriched in the primary component is passed into a thermal swing
adsorption system, and a stream which is further enriched in the primary
component withdrawn therefrom and passed into a pressure swing adsorption
system. A final product stream which is still further enriched in the
primary component is withdrawn from the pressure swing adsorption system.
The permeable adsorbent membrane used in the first and second membrane
separation zones comprises adsorbent material which preferentially adsorbs
the secondary component or components in the gas mixture being separated.
The adsorbent material is selected from the group consisting of activated
carbon, zeolite, activated alumina, silica, and combinations thereof. The
same adsorbent material can be used in each membrane separation zone, or
alternatively a different adsorbent material can be used in each membrane
separation zone.
The invention is also an improvement in the method of separating a
multicomponent gas mixture containing at least one primary component and
at least one secondary component into a product stream enriched in the
primary component and one or more additional product streams enriched in
the secondary component, wherein the method includes:
(a) introducing a feed gas mixture comprising the primary and secondary
components into a first membrane separation zone having a gas permeable
membrane which divides the zone into a feed side and a permeate side,
wherein the gas passes through the feed side of the membrane separation
zone and a portion of the secondary component selectively permeates
through the membrane to the permeate side;
(b) withdrawing from the feed side of the first membrane separation zone a
first nonpermeate stream as the product stream enriched in the primary
component;
(c) withdrawing from the permeate side of the first membrane separation
zone a permeate stream which is enriched in the secondary component;
(d) introducing at least a portion of the permeate stream as a feed gas
into a second membrane separation zone having a gas permeable membrane
comprising adsorbent material which divides the zone into a feed side and
a permeate side, wherein the feed gas passes through the feed side of the
membrane separation zone and a portion of the secondary component
selectively permeates through the membrane to the permeate side, and
withdrawing therefrom an additional product stream enriched in the
secondary component; and
(e) withdrawing from the feed side of the second membrane separation zone a
second nonpermeate stream which is further enriched in the primary
component, and combining at least a portion of the second nonpermeate
stream with the multicomponent gas mixture to provide the feed gas to the
first membrane separation zone.
The improvement of the present invention comprises utilizing an adsorbent
membrane comprising adsorbent material as the gas permeable membrane in
each of the first and second membrane separation zones in which a portion
of the secondary component is selectively adsorbed from the feed gas to
each zone by the adsorbent material and permeates through the adsorbent
membrane to the permeate side of each zone. The first and second membrane
separation zones are operated such that the ratio of the recovery of the
primary component in the first membrane separation zone to the recovery of
the primary component in the second membrane separation zone is less than
about 1.0, and preferably less than about 0.8, wherein the recovery of the
primary component in any membrane separation zone is defined as the molar
flow rate of the primary component in the nonpermeate stream withdrawn
from the zone divided by the molar flow rate of the primary component in
the feed gas introduced into the zone.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
FIG. 1 is a schematic flow diagram which includes a two-stage adsorbent
membrane separation system of the present invention.
FIG. 2 is a plot of hydrocarbon rejection vs hydrogen recovery for the
separation of a refinery fluid catalytic cracker offgas using a tubular
adsorbent membrane of the present invention.
FIG. 3 is a plot of CO.sub.2 and CH.sub.4 rejection vs hydrogen recovery
for the separation of offgas from a pressure swing adsorption system of a
steam-methane reforming hydrogen production system using a tubular
adsorbent membrane of the present invention.
FIG. 4 is a plot of hydrogen sulfide rejection vs hydrogen recovery for the
separation of a hydrogen-hydrogen sulfide mixture using a tubular
adsorbent membrane of the present invention.
FIG. 5 is a plot of the overall hydrogen recovery vs the hydrogen stage
recovery ratio for the separation of a hydrogen-carbon dioxide-methane
mixture using a two-stage tubular adsorbent membrane of the present
invention.
FIG. 6 is a plot of the carbon dioxide concentration in the hydrogen
product vs the hydrogen stage recovery ratio for the separation of a
hydrogen-carbon dioxide-methane mixture using a two-stage tubular
adsorbent membrane of the present invention.
FIG. 7 is a plot of the overall hydrogen recovery vs the hydrogen stage
recovery ratio for the separation of a hydrogen-hydrogen sulfide mixture
using a two-stage tubular adsorbent membrane of the present invention.
FIG. 8 is a plot of the hydrogen sulfide concentration in the hydrogen
product vs the hydrogen stage recovery ratio for the separation of a
hydrogen-hydrogen sulfide mixture using a two-stage tubular adsorbent
membrane system of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Staging improves the performance of permeable membrane systems by
increasing either or both the recovery and purity of the product
components. When the nonpermeate stream from a membrane module or stage
contains the desired product component, product purity and recovery can be
increased by passing the permeate stream to a second membrane module or
stage and recycling the nonpermeate stream from this second membrane stage
to the feed of the first membrane stage. The product is recovered as the
nonpermeate stream from the first membrane stage and the removed
impurities are withdrawn as the permeate stream from the second membrane
stage.
The use of staged adsorbent membranes in this configuration is of
particular utility in separating gas mixtures in which the primary product
is obtained in the nonpermeate stream at near feed pressure. This
configuration is preferred for example in the recovery of hydrogen from
mixtures of hydrogen and higher molecular weight components such as carbon
oxides, hydrogen sulfide, ammonia, and hydrocarbons having up to about
five carbon atoms. Other gases amenable to separation by this method
include mixtures of helium and natural gas components as well as the
recovery of methane from mixtures of methane with impurities such as
carbon oxides, hydrogen sulfide, and heavier hydrocarbons.
In the present specification, the term "stage" is defined as an adsorbent
membrane module comprising an assembly of one or more adsorbent membranes
contained within a vessel having one or more feed gas inlets, a high
pressure gas outlet, one or more low pressure permeate gas outlets, and
optionally a low pressure sweep gas inlet. Each of the adsorbent membranes
contained in the vessel can be supported on a substrate having a tubular,
flat sheet, or monolith configuration as described below.
The use of staged adsorbent membranes of the present invention for such
hydrogen-containing gas mixtures improves the recovery and purity of the
hydrogen product and allows recovery of the hydrogen product at near feed
pressure. This latter feature is especially beneficial when the hydrogen
product is further purified by pressure or thermal swing adsorption as
later described.
The present invention is illustrated in FIG. 1. Fresh feed gas mixture 1 is
combined with recycle stream 3 (later defined) to form net feed 5 which is
introduced at a pressure of about 30 to about 1500 psia into adsorbent
membrane separation zone 7 which is divided into feed side 9 and permeate
side 11 by adsorbent membrane 13. This adsorbent membrane comprises
adsorbent material supported by a porous substrate in which the adsorbent
material is a coating on the surface of the substrate. Alternatively, some
or all of the adsorbent material is contained within the pores of the
substrate. Adsorbent membrane separation zone 7 as shown in FIG. 1 is
representative of an adsorbent membrane stage as defined above and is the
first of two stages as described below.
Fresh feed gas mixture 1 contains at least one primary component and at
least one secondary component, wherein the secondary component is more
strongly adsorbed on the adsorbent material used and is more permeable
through adsorbent membrane 13, while the primary component is less
strongly adsorbed on the adsorbent material and is less permeable through
adsorbent membrane 13. Additional primary and secondary components may be
present, each of which has a different strength of adsorption and
permeability through adsorbent membrane 13. More strongly adsorbed
components with higher permeability can inhibit the permeability of less
strongly adsorbed components, which improves the relative separation of
desired product components and undesired impurity components.
The adsorbent material can be selected from activated carbon, zeolite,
activated alumina, silica, and mixtures or combinations thereof. Activated
carbon is a preferred adsorbent material for the separation of hydrogen
from mixtures containing hydrogen and hydrocarbons, carbon oxides,
ammonia, or hydrogen sulfide. The characteristics and methods of
preparation of representative activated carbon adsorbent membranes are
described in U.S. Pat. No. 5,104,425 which incorporated herein by
reference. A representative type of membrane is made by coating a porous
graphite substrate with a thin film of an aqueous suspension (latex)
containing a polyvinylidine chloride polymer, drying the coated substrate
at 150.degree. C. for five minutes, heating the substrate in nitrogen to
600.degree.-1000.degree. C. at a rate of 1.degree. C. per minute, holding
at temperature for three hours, and cooling to ambient temperature at
1.degree.-10.degree. C. per minute. The polymer coating is carbonized
during the heating step thereby forming an ultrathin layer of microporous
carbon on the substrate. Other polymers can be used for coating prior to
the carbonization step provided that these polymers can be carbonized to
form the required porous carbon adsorbent material. Such alternate
polymers can be selected from polyvinyl chloride, polyacrylonitrile,
styrene-divinylbenzene copolymer, and mixtures thereof. Alternatively, the
substrate can be a porous ceramic material. The substrate serves as a
support for the adsorptive material and typically the substrate has
essentially no effect on the separation of the gas mixture.
The adsorbent membrane and substrate can be fabricated in a tubular
configuration in which the microporous adsorbent material is deposited on
either the inner or the outer surface of a tubular porous substrate, and
the resulting tubular adsorbent membrane elements can be assembled in a
shell-and-tube configuration in an appropriate pressure vessel to form a
membrane module. Alternatively, the adsorbent membrane and support can be
fabricated in a flat sheet configuration which can be assembled into a
module using a plate-and-frame arrangement. Alternatively, the adsorbent
membrane and support can be fabricated in a monolith or multichannel
configuration to provide a high membrane surface area per unit volume of
membrane module. The support material of the tubular membrane, the flat
sheet membrane, or the monolith can be a porous ceramic, porous glass,
porous metal, or a porous carbon material. A hollow fiber configuration
may be used in which the adsorbent membrane is supported by fine hollow
fibers of the substrate material. A plurality of membrane modules in
parallel or series can be utilized when gas feed rates and separation
requirements exceed the capability of a single module of practical size.
In each of these configurations, the membrane zone is divided into a feed
side and permeate side as illustrated in schematic fashion by membrane
separation zone 7 in FIG. 1.
Net feed 5 at a pressure of about 30 to about 1500 psia is passed through
feed side 9 of membrane separation zone 7, and portions of the more
strongly adsorbed secondary components contained therein selectively
adsorb and permeate through adsorbent membrane 13 by the dominant
mechanism of selective surface flow in the adsorbed phase. The resulting
separation yields permeate 15 which is enriched in the secondary
components and nonpermeate 17 which is enriched in the primary component.
Nonpermeate stream 17 is at a pressure slightly less than feed 5 due to
the small pressure drop through feed side 9 while permeate 15 is at a
significantly lower pressure, typically between 2 and 25 psia.
Permeate 15 typically contains a significant concentration of valuable
primary components which preferably should be recovered in addition to the
primary components in nonpermeate stream 17. This is accomplished by
compressing permeate 15 if necessary to a pressure of 30 to 1500 psia in
compressor 19 and introducing compressed feed 21 into adsorbent membrane
separation zone 23 which is divided into feed side 25 and permeate side 27
by adsorbent membrane 29. A portion 22 of permeate 15 from membrane
separation zone 7 optionally can be withdrawn prior to membrane separation
zone 23, either before or after compressor 19.
Portions of the more strongly adsorbed secondary components in compressed
feed 21 selectively adsorb and permeate through adsorbent membrane 29 by
the dominant mechanism of selective surface flow in the adsorbed phase.
The resulting separation yields permeate 31 which is further enriched in
the secondary components and is withdrawn as a waste or byproduct stream.
Nonpermeate 33 is enriched in the primary components and is at a pressure
slightly less than compressed feed 21 due to the small pressure drop
through feed side 25, while permeate 31 is at a significantly lower
pressure, typically between 2 and 25 psia.
Nonpermeate 33 optionally is compressed in compressor 35 to provide recycle
stream 3 which is combined with fresh feed 1 to provide net feed 5 which
is processed as described earlier. This recycle of nonpermeate 33
significantly increases the recovery of primary components in nonpermeate
17 from membrane separation zone 7. Optionally, a portion 37 of
nonpermeate 33 can be withdrawn prior to recycle, either before or after
compressor 35.
Adsorbent membrane separation zones 7 and 23 typically are operated at
ambient temperatures, but if desired may be operated at temperatures as
low as -100.degree. C. or as high as 150.degree. C.
The same adsorbent material may be used in adsorbent membranes 13 and 29,
and these adsorbent membranes typically have the same or similar gas
separation characteristics. If desired, the adsorbent membranes may be
made of the same adsorbent material (e.g. activated carbon) but with
different pore size distributions in each zone. Alternatively, different
adsorbent materials having different separation characteristics can be
utilized in adsorbent membranes 13 and 29 if desired to optimize the
separation performance of adsorbent membrane separation zones 7 and 23. If
desired, either or both of the membranes in the two separation zones can
be polymeric membranes.
The invention as described above preferably utilizes two adsorbent membrane
separation zones or stages, but additional stages can be utilized if
extremely high product purity or recovery is required.
In the operation of a single-stage gas-permeable membrane for the
separation of gas mixtures, the degree of recovery of the desired product
component and the purity of the product component are related such that
higher recovery is achieved only at lower purity, and conversely higher
product purity is attained only at the expense of decreased recovery. This
inverse relationship occurs for polymeric membranes and porous diffusion
membranes as is known in the art, and has been found to occur as well for
adsorbent membranes associated with the present invention as described in
the following Examples. When membranes are arranged in multiple stages,
the relationship between product recovery and purity becomes more complex.
The overall product recovery and overall product purity in a multi-stage
membrane system are functions of the staging configuration and the
operating conditions of each individual stage.
It has been discovered for the staging configuration described above for
the present invention, in which the desired product component is recovered
in the nonpermeate stream from the first stage of a two-stage adsorbent
membrane system, that a preferred operating mode exists for each stage
which results in the most favorable tradeoff between overall product
recovery and overall product purity for the two-stage system. In the
preferred operating mode, the first and second membrane stages are
operated such that the ratio of the recovery of the primary component in
the first stage to the recovery of the primary component in the second
stage is less than about 1.0 and preferably less than about 0.8. This
ratio of the primary component recoveries is defined as the stage recovery
ratio for that component. Recovery of a primary component in any given
stage is defined as the fraction or percentage of the primary component in
the feed to that stage which is recovered in the nonpermeate product
stream from that stage. Preferably the recovery of the primary component
in the first stage is less than about 75%. Referring to FIG. 1, the
preferred mode of operation is that the stage recovery ratio for adsorbent
membrane separation zones 7 and 23 is less than about 1.0 and preferably
less than about 0.8. The recovery of the primary component in adsorbent
membrane separation zone 7 is less than about 75%.
Nonpermeate 17, which is enriched in the primary component and contains one
or more residual secondary components, optionally is further enriched by
introducing the nonpermeate at about 100 to about 500 psia into pressure
swing adsorption (PSA) system 43. If feed 5 to the adsorbent membrane zone
7 is below 100 psia, compressor 39 is utilized to provide compressed
nonpermeate 41 as feed to PSA system 43. High purity primary product 45
and waste stream 47 are withdrawn therefrom. Adsorption system 43 can be a
pressure swing adsorption system (PSA) of a type known in the art;
alternatively, a thermal swing adsorption system of a type known in the
art can be utilized in place of a PSA system if nonpermeate 17 contains
dilute residual secondary components which are strongly adsorbed on the
adsorbent utilized in adsorption system 43.
The adsorbent membrane and staging configuration of the present invention
are particularly well suited for the purification and recovery of low
molecular weight primary components such as hydrogen, helium, or methane
from mixtures which also contain higher molecular weight secondary
components such as carbon oxides, water, hydrogen sulfide, ammonia, and
hydrocarbons. A beneficial characteristic of adsorbent membranes used in
such separations is that the heavier contaminant components selectively
permeate through the membrane and the lighter primary component is
recovered as a nonpermeate at near feed pressure. Recovery of the primary
component at relatively high purity, at acceptable recovery, and at high
pressure is particularly advantageous when final purification of the
primary component by pressure or thermal swing adsorption is required.
As illustrated in the Examples which follow, operation of the two-stage
adsorbent membrane system in this preferred mode allows the overall
recovery of the primary component to be increased with no penalty or a
relatively small penalty in decreased product purity. When the two-stage
adsorbent membrane system is operated at conditions outside of this
preferred mode, however, increases in overall product recovery are
accompanied by more significant decreases in product purity. In the
preferred mode of the invention (i.e. at a stage recovery ratio of less
than about 1.0), a higher product purity can be attained for a given
overall product recovery compared with the purity attained by operating
outside the preferred mode (i.e. at a stage recovery ratio of greater than
1.0).
The recovery of the primary component in each stage of the two-stage system
can be controlled by proper selection of the design and operating
conditions for each stage. An important design parameter which determines
the recovery of the primary component in a given stage is the ratio of the
effective membrane surface area (A) to the feed flow rate (F) to that
stage, commonly designated as A/F. In a given stage, a lower value of A/F
yields a higher recovery and lower purity, while conversely a higher value
of A/F yields a higher purity and lower recovery. The separation
properties of the adsorbent membrane material, namely selectivity and
permeability, also will affect the primary product recovery. Adsorbent
membranes having different separation properties can be used in each stage
if desired. The ratio of feed pressure to permeate pressure also will
affect primary product recovery in each stage, as will the stage operating
temperature. These design and operating parameters can be selected for
each stage to give the desired stage recovery ratio for the present
invention.
EXAMPLE 1
An adsorbent membrane was prepared by coating the bore side of a
macroporous alumina support tube having a length of 30 cm, an internal
diameter of 0.56 cm, and a wall thickness of 0.165 cm with a thin uniform
layer of polyvinilidene chloride-acrylate terpolymer latex which contained
polymer beads having diameters of 0.1-0.14 .mu.m in an aqueous emulsion
containing 4 wt % solids. The coating was dried under nitrogen at
50.degree. C. and the coated tube was heated to 600.degree. C. under
nitrogen purge to carbonize the coating and form a layer of adsorbent
carbon. The completed tubular membrane was mounted in a pressure vessel
shell such that pressurized feed gas could be introduced into the bore
side and recovered at near feed pressure from the bore side, while
permeate gas could be collected from the shell side in a countercurrent
mode at low pressure. Pressures and flow rates of the feed, non-permeate,
and permeate gas streams were measured by the usual laboratory methods.
The tubular adsorbent membrane was then subjected to laboratory testing for
the separation of a mixture representative of an offgas from a petroleum
refinery fluid catalytic cracking (FCC) unit which contained 20% hydrogen,
20% methane, 16% ethane plus ethylene, and 44% propane plus propylene on a
molar basis.
The results of the tests are plotted in FIG. 2 which shows the performance
characteristics of the tubular adsorbent membrane in a plot of hydrocarbon
rejection vs. hydrogen recovery for each of the hydrocarbons in the
mixture. A plot of A/F vs. hydrogen recovery is also included.
EXAMPLE 2
The performance data of Example 1 were used to estimate the performance of
the two-stage adsorbent membrane system of FIG. 1. The system is operated
at a feed pressure of 3.0 atm (abs) and ambient temperature, with a first
stage hydrogen recovery of 55% and hydrocarbon rejections of 98.8% for
propane plus propylene, 95% for ethane plus ethylene and 57.5% for
methane. The second stage operates at a 70% hydrogen recovery and
hydrocarbon rejections of 96.6%, 89.4%, and 35.6% for propane plus
propylene, ethane plus ethylene, and methane respectively. The first stage
yields a nonpermeate product stream containing 51.8% hydrogen, 43.5%
methane, 2.9% ethane plus ethylene, and 1.8% propane plus propylene (molar
basis). The overall hydrogen recovery for the two-stage system is 80.3%. A
stream and operating summary for the two-stage system is given in Table
TABLE 1
______________________________________
Two-Stage Membrane System with FCC Offgas
______________________________________
Stream No.
Flow Composition, Mole %
(FIG. 1)
MM SCFD H.sub.2
CH.sub.4
C.sub.2 H.sub.4 + C.sub.2 H.sub.6
C.sub.2 H.sub.4 + C.sub.2 H.sub.8
______________________________________
1 1.00 20.0 20.0 16.0 44.0
3 0.24 37.9 48.4 7.4 6.3
5 1.24 23.5 25.6 14.3 36.6
15 0.93 14.1 19.6 18.1 48.2
17 0.31 51.8 43.5 2.9 1.8
31 0.69 5.7 9.4 21.9 63.0
______________________________________
Stage 1 Stage 2 Overall
______________________________________
H.sub.2 Recovery, %
55.0 70.0 80.3
CH.sub.4 Rejection, %
57.0 35.6 32.5
Ethane and Ethylene Rejection, %
95.0 89.4 94.4
Propane and Propylene Rejection, %
98.8 96.6 98.7
Relative Membrane Area
0.59 0.41 1.0
______________________________________
EXAMPLE 3
The tubular adsorbent membrane of Example 1 was then subjected to
laboratory testing for the separation of a mixture representative of an
offgas from a steam-methane reformer pressure swing adsorption (PSA)
system which contained 35% hydrogen, 10% methane, and 55% carbon dioxide
on a molar basis. The testing and data reduction was conducted in the same
manner as Example 1 except that the feed pressure was 4.0 atm (abs) and no
sweep gas was used.
The results of the tests are plotted in FIG. 3 which shows the performance
characteristics of the tubular adsorbent membrane in a plot of methane and
carbon dioxide rejection vs. hydrogen recovery. A plot of A/F vs. hydrogen
recovery is also included.
EXAMPLE 4
The performance data of FIG. 3 were used to calculate the performance of
the two-stage adsorbent membrane system of FIG. 1. The system is operated
at a feed pressure of 4.0 atm (abs) and ambient temperature, with a first
stage hydrogen recovery of 40%, and with carbon dioxide and methane
rejections of 91.9% and 80.6% respectively. The second stage operates at a
65% hydrogen recovery with carbon dioxide and methane rejections of 73.4%
and 58.1% respectively. The first stage yields a nonpermeate product
stream containing 72.2% hydrogen, 9.2% methane, and 18.6% carbon dioxide
(molar basis). The overall hydrogen recovery for the two-stage system is
66%. A stream and operating summary for the two-stage system is given in
Table
TABLE 2
______________________________________
Two-Stage Membrane System with PSA Offgas
______________________________________
Stream No. Flow, Composition, Mole %
(FIG. 1) MM SCFD Hydrogen Methane
CO.sub.2
______________________________________
1 1.00 35.0 10.0 55.0
3 0.45 49.4 11.3 39.3
5 1.43 39.5 10.4 50.1
15 1.13 30.3 10.7 59.0
17 0.32 72.2 9.2 18.6
31 0.68 17.7 10.3 72.0
Stage 1 Stage 2 Overall
______________________________________
H.sub.2 Recovery, %
40.0 65.0 65.6
CH.sub.4 Rejection, %
80.6 58.1 70.7
CO.sub.2 Rejection, %
91.9 73.4 89.3
Relative Membrane Area
0.63 0.37 1.0
______________________________________
EXAMPLE 5
The performance data of FIG. 3 were used to calculate and compare the
overall hydrogen product recovery and purity for the two-stage system as a
function of the individual hydrogen recoveries of stages 1 and 2. The
stage recovery ratio for hydrogen, defined as the hydrogen recovery in the
first stage divided by the hydrogen recovery in the second stage, was
calculated at a constant hydrogen recovery of 40% in the first stage and
for hydrogen recoveries of 40% to 70% in the second stage. The overall
hydrogen recovery for the two-stage system was determined at each value of
the calculated hydrogen stage recovery ratio. The hydrogen stage recovery
ratio then was estimated at a constant hydrogen recovery of 40% in the
second stage and for hydrogen recoveries of 40% to 70% in the first stage,
and again the overall hydrogen recovery for the two-stage system was
determined at each value of the calculated hydrogen stage recovery ratio.
These results are plotted in FIG. 4.
The performance data of FIG. 3 were used to determine the hydrogen product
purity at each of the calculated values of the hydrogen stage recovery
ratio given above. Hydrogen product purity is represented by the residual
concentration of carbon dioxide in the product gas. These results are
plotted in FIG. 5.
FIGS. 4 and 5 in combination illustrate an important feature of the
invention, as described in general terms earlier in this specification, in
which a preferred operating mode exists for each stage which results in
the most favorable tradeoff between overall hydrogen product recovery and
overall hydrogen product purity for the two-stage system. In the preferred
operating mode, the first and second membrane stages are operated such
that the hydrogen stage recovery ratio is less than about 1.0 and
preferably less than about 0.8. It is seen from FIGS. 4 and 5 that below a
hydrogen stage recovery ratio of about 1.0, increasing the overall
hydrogen recovery by reducing the hydrogen stage recovery ratio results in
a very slight increase in hydrogen product purity. For example, if the
hydrogen stage recovery ratio is reduced from about 1.0 to about 0.62, the
overall hydrogen recovery increases from about 53% to about 66% (FIG. 4)
while the concentration of carbon dioxide in the hydrogen product actually
drops slightly from about 18.5 mole % (FIG. 5). In contrast, increasing
the overall hydrogen recovery by increasing the hydrogen stage recovery
ratio above about 1.0 results in a decrease in hydrogen product purity.
For example, if the hydrogen stage recovery ratio is increased from about
1.0 to about 1.38, the overall hydrogen recovery increases from about 53%
to about 67% (FIG. 4) but the concentration of carbon dioxide in the
hydrogen product increases from about 18.5 mole % to about 26 mole % (FIG.
5).
Thus the most favorable operating region for the two-stage adsorbent
membrane system occurs below a hydrogen stage recovery ratio of about 1.0
wherein the hydrogen recovery in the first stage is lower, and preferably
significantly lower, than the hydrogen recovery in the second stage.
Operating the two-stage system above a hydrogen recovery ratio of about
1.0 can yield an acceptable overall recovery of hydrogen, but with a
penalty of decreased hydrogen product purity. This is illustrated by an
example wherein an overall hydrogen recovery of 65% is required. From FIG.
4 it is seen that this recovery can be achieved at a hydrogen stage
recovery ratio of either about 0.68 or about 1.32. From FIG. 5 it is seen
that a hydrogen stage recovery ratio of about 0.68 yields a hydrogen
product containing about 18.5 mole % carbon dioxide, while a hydrogen
stage recovery ratio of about 1.32 yields a hydrogen product containing
about 27 mole % carbon dioxide.
EXAMPLE 6
The tubular adsorbent membrane of Example 1 was subjected to laboratory
testing for the separation of a hydrogen-hydrogen sulfide mixture which
contained 50% hydrogen and 50% hydrogen sulfide on a molar basis at a
pressure of 115 psia. The testing used the same procedures as Example 1.
The results of the tests are plotted in FIG. 4 which shows the performance
characteristics of the tubular adsorbent membrane in a plot of hydrogen
sulfide rejection vs. hydrogen recovery for the mixtures and conditions
tested.
EXAMPLE 7
The performance data of FIG. 4 were used to estimate the performance of the
two-stage adsorbent membrane system of FIG. 1 with an equimolar
hydrogen-hydrogen sulfide feed mixture. The system is operated at a feed
pressure of 115 psia and ambient temperature with a first stage hydrogen
recovery of 30% and a hydrogen sulfide rejection of 99.1%. The second
stage operates at a 90% hydrogen recovery with a hydrogen sulfide
rejection of 60.3%. The first stage yields a nonpermeate product stream
containing 98.0% hydrogen and 2.0% hydrogen sulfide(molar basis). The
overall hydrogen recovery for the two-stage system is 77% and the overall
hydrogen sulfide rejection is 98.3%. A stream and operating summary for
the two-stage system is given in Table
TABLE 3
______________________________________
Two-Stage Membrane System with Hydrogen-Hydrogen Sulfide
______________________________________
Mixture
Stream No.
Flow, Composition, Mole %
(FIG. 1) MM SCFD Hydrogen Hydrogen Sulfide
______________________________________
1 1.00 50.0 50.0
3 1.18 72.2 27.8
5 2.18 62.0 38.0
15 1.77 53.6 46.4
17 0.41 98.0 2.0
31 0.59 16.1 83.9
______________________________________
Stage 1 Stage 2 Total
______________________________________
Hydrogen Recovery, %
30 90 77
Hydrogen Rejection, %
99.1 60.3 98.3
Relative Membrane Area
0.81 0.19 1.0
______________________________________
EXAMPLE 8
The performance data of FIG. 4 were used to estimate and compare the
overall hydrogen product recovery and purity for the two-stage system as a
function of the individual hydrogen recoveries of stages 1 and 2. The
stage recovery ratio for hydrogen, defined as the hydrogen recovery in the
first stage divided by the hydrogen recovery in the second stage, was
calculated at a constant hydrogen recovery of 30% in the first stage with
hydrogen recoveries of 30% to 90% in the second stage. The overall
hydrogen recovery for the two-stage system was determined at each value of
the calculated hydrogen stage recovery ratio. The hydrogen stage recovery
ratio then was calculated at a constant hydrogen recovery of 30% in the
second stage with hydrogen recoveries of 30% to 90% in the first stage,
and again the overall hydrogen recovery for the two-stage system was
determined at each value of the calculated hydrogen stage recovery ratio.
These results are plotted in FIG. 7.
The performance data of FIG. 6 were used to determine the hydrogen product
purity at each of the calculated values of the hydrogen stage recovery
ratio above, and the results are plotted in FIG. 8. Hydrogen product
purity is defined by the residual concentration of hydrogen sulfide in the
product gas.
FIGS. 7 and 8 in combination illustrate an important feature of the
invention, as described in general terms earlier in this specification, in
which a preferred operating mode exists for each stage which results in
the most favorable tradeoff between overall hydrogen product recovery and
overall hydrogen product purity for the two-stage system. In the preferred
operating mode, the first and second membrane stages are operated such
that the hydrogen stage recovery ratio is less than about 1.0 and
preferably less than about 0.8. It is seen from FIGS. 7 and 8 that below a
hydrogen stage recovery ratio of about 1.0, increasing the overall
hydrogen recovery by reducing the hydrogen stage recovery ratio results in
a slight increase in hydrogen product purity. For example, if the hydrogen
stage recovery ratio is reduced from about 1.0 to about 0.5, the overall
hydrogen recovery increases from about 38% to about 52% (FIG. 7) while the
mole fraction of hydrogen sulfide in the hydrogen product drops slightly
from about 0.025 to about 0.020 (FIG. 8). In contrast, increasing the
overall hydrogen recovery by increasing the hydrogen stage recovery ratio
above about 1.0 results in a decrease in hydrogen product purity. For
example, if the hydrogen stage recovery ratio is increased from about 1.0
to about 1.5, the overall hydrogen recovery increases from about 38% to
about 54% (FIG. 7) while the mole fraction of hydrogen sulfide in the
hydrogen product increases from about 0.025 to about 0.045 (FIG. 8).
Thus the most favorable operating region for the two-stage adsorbent
membrane system occurs below a hydrogen stage recovery ratio of about 1.0
wherein the hydrogen recovery in the first stage is lower, and preferably
significantly lower, than the hydrogen recovery in the second stage.
Operating the two-stage system above a hydrogen recovery ratio of about
1.0 can yield an acceptable overall recovery of hydrogen, but with a
penalty of decreased hydrogen product purity. This is illustrated by an
example wherein an overall hydrogen recovery of 70% is required. From FIG.
7 it is seen that this recovery can be achieved at a hydrogen stage
recovery ratio of either about 0.4 or about 2.05. From FIG. 8 it is seen
that a hydrogen stage recovery ratio of about 0.4 yields a hydrogen
product containing slightly less than about 0.02 mole fraction hydrogen
sulfide, while a hydrogen stage recovery ratio of about 2.05 yields a
hydrogen product containing about 0.06 mole fraction hydrogen sulfide.
A generic definition of the preferred operating region for the present
invention is that region which exhibits a large slope in the curve of
overall hydrogen recovery vs. hydrogen stage recovery ratio (i.e. FIGS. 4
and 7) and a small slope in the curve of hydrogen product impurity level
vs. hydrogen stage recovery ratio (i.e. FIGS. 5 and 8).
EXAMPLE 9
The results of Examples 2, 4, and 7 were compared to illustrate the
preferred operating range of the present invention. These results are
summarized in Table
TABLE 4
______________________________________
Comparison of Operating Results for Examples 2, 4, and 7
Feed Gas
FCC
Hydrogen Recovery, %
Offgas PSA Offgas
H.sub.2 --H.sub.2 S Mix
______________________________________
Stage 1 55 40 30
Stage 2 70 65 90
Overall 80 66 77
Hydrogen Stage Recovery Ratio
0.79 0.61 0.33
Hydrogen Product Purity,
51.8 72.2 98.0
Mole %
______________________________________
These Examples for three different gas mixtures illustrate the preferred
operating mode for separation by the two-stage adsorbent membrane system
defined in FIG. 1, namely, hydrogen stage recovery ratios of less than
about 1.0 and preferably less than about 0.8. Operation of the two-stage
adsorbent membrane system in this preferred mode allows the overall
recovery of the primary component to be increased with no penalty or a
very small penalty in decreased product purity. When the two-stage
adsorbent membrane system is operated at conditions outside of this
preferred mode, however, increases in overall product recovery are
accompanied by more significant decreases in product purity. In the
preferred mode of the invention (i.e. at a stage recovery ratio of less
than about 1.0), a higher product purity can be attained for a given
overall product recovery than can be attained by operating outside the
preferred mode (i.e. at a stage recovery ratio of greater than 1.0).
The adsorbent membrane and staging configuration of the present invention
are particularly well suited for the purification and recovery of low
molecular weight primary components such as hydrogen, helium, or methane
from mixtures which also contain higher molecular weight secondary
components such as carbon oxides, water, hydrogen sulfide, ammonia,
nitrogen, and hydrocarbons. A beneficial characteristic of adsorbent
membranes used in such separations is that the heavier contaminant
components selectively permeate through the membrane and the lighter
primary component is recovered as a nonpermeate at near feed pressure.
Recovery of the primary component at relatively high purity, at acceptable
recovery, and at high pressure is particularly advantageous when final
purification of the primary component by pressure or thermal swing
adsorption is required.
The essential characteristics of the present invention are described
completely in the foregoing disclosure. One skilled in the art can
understand the invention and make various modifications without departing
from the basic spirit of the invention, and without deviating from the
scope and equivalents of the claims which follow.
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